Freshman Sen. Rand Paul has long been worried about a sea of red ink drowning the nation’s finances.

But now he may be seeing red over his own personal finances.

The Kentucky Republican, an ophthalmologist for nearly two decades, has been performing free eye surgeries on his patients since he became a senator in January — in order to circumvent the Senate’s conflict-of-interest rules that prohibit senators from being compensated from outside employment. That has left the freshman favorite of the tea party footing the bill for his hefty liability insurance and expensive surgical equipment, running up a tab of tens of thousands of dollars in expenses.

Paul is quietly lobbying his colleagues to grant him an exemption from Senate rules so he can bill his patients strictly to cover his expenses — but not take a salary of his own. The 48-year-old senator says he needs to continue performing surgeries because if he drops it altogether, he’ll lose the skills he’d need to return to private practice once he stops serving as a senator. And he says he wants to take the fight to the Senate floor, enlisting veteran Sen. Orrin Hatch (R-Utah) and Ethics Committee member Sen. Jim Risch (R-Idaho) to help with his cause.

“I think for them to take a step like this, it’s going to take somebody pushing them,” Paul told POLITICO in the Capitol.

The fight highlights the growing pains of a freshman senator — who has never before served in elected office — adapting to the iron-clad rules of the traditional institution. And it’s reminiscent of an unsuccessful effort by another conservative Republican, Tom Coburn of Oklahoma, who tried to change Senate rules to grant a similar exception in 2005. As a House member, Coburn — an obstetrician — was allowed to practice medicine if he did not personally profit, allowing him to deliver some 400 babies during his six years as a congressman.

But the Senate grants no exception, and the full Senate rejected Coburn’s effort to make such a change in 2005. The Ethics Committee has since privately told him that he can deliver babies at not-for-profit hospitals — and not receive compensation to cover his expenses, his office said.

“It’s disappointing Congress, through the Ethics Committee, has done its best to discourage citizen legislators from serving,” said John Hart, spokesman for Coburn.

Supporters said the rule is a common-sense practice to prevent even the perception that there could be a conflict of interest. They said a senator could be subjected to back-channel lobbying by a special interest if compensation were allowed. Moreover, if Paul were given his exception, they said, senators would be lining up to win a carve-out of their own. A rules change would require support from two-thirds of the Senate.

“I think it’s a slippery slope” if Paul were to win an exemption, said Melanie Sloan, head of the watchdog group Citizens for Responsibility and Ethics in Washington.

The Senate also may not have much appetite to open up a debate on ethics, especially since Nevada Sen. John Ensign resigned his seat amid charges that he violated Senate rules during a sex scandal.

In Paul’s case, he met with the Ethics Committee soon after he was sworn in to see whether he could bill his patients while taking no salary. He was told the rules do not allow such an arrangement and that his name cannot be affiliated with an organization that is receiving such compensation.

Paul said he successfully lobbied a committee member, Risch, to write a letter in support of granting him an exception — but has yet to hear officially in writing from the committee or its chairwoman, Sen. Barbara Boxer (D-Calif.).

“I’m going to do some surgery this weekend at home for free,” Paul said last week. “But there’s only so much you can do for free, because nobody wants you to do it too much for free because it costs them money.”

As a senator, Paul will make an annual salary of $174,000; he reported in his financial disclosure papers to the Secretary of the Senate last year that his practice in Bowling Green yielded an income of $100,000 to $1 million. That makes him far less wealthy than most senators.

Paul says the situation for physicians is far different than for other professions — like lawyers — where red flags would be raised if a senator were to do pro bono work for a corporate client.

Hatch, a lawyer by training, seems to agree — and is drafting a rules change to allow physicians who serve in the Senate to continue practicing on a limited basis.

“I want people like [them] to keep their professional licenses and keep their professional abilities,” Hatch said in an interview. “I think that’s an advantage to the Senate.”

Still, one of the physicians serving as a senator hasn’t taken the fight as far as Paul or Coburn.

Republican Sen. John Barrasso, who spent 24 years as an orthopedic surgeon in Wyoming, dispenses free medical advice to senators — as he did to Senate Majority Leader Harry Reid (D-Nev.) after he dislocated his shoulder earlier this month after a fall.